Generally, in a manufacturing process of semiconductor integrated circuit, the integrated circuit is formed on a target substrate, for example, a semiconductor wafer (hereinafter, simply referred to as a “wafer”) by repeatedly performing various kinds of processes, such as a film forming process, an etching process, a thermal process, and the like, on the wafer. Further, a preset post process can also be performed on the wafer after the completion of each process. The post process may be, for example, a process for cleaning the wafer (for example, a process for removing deposited substances stuck on the wafer, or the like) and a process for measuring a processing result of each process (for example, a film thickness measuring process, a particle measuring process, or the like).
Such wafer processes are performed by a substrate processing apparatus including a processing chamber configured to perform a preset process such as a plasma process and a measuring process therein. The substrate processing apparatus also includes, for example, a transfer robot having a transfer arm for transferring the wafer, wherein the transfer arm is configured to be rotatable and movable back and forth. The wafer is transferred into the processing chamber by the transfer arm. In general, a mounting table for mounting the wafer thereon is installed in the processing chamber, and the wafer is transferred between the mounting table and the transfer arm.
However, in order to perform a proper process on the wafer transferred onto the mounting table, the wafer needs to be placed on the mounting table accurately without being deviated in horizontal direction. For the purpose, it is required to detect a positional deviation of the wafer first and to perform a positioning of the wafer after correcting it when the positional deviation is detected.
In this regard, the positional deviation of the wafer has been conventionally detected by detecting the entire circumference of the peripheral portion of the wafer while rotating the wafer at least one time. For example, Patent Document 1 discloses a method of detecting a positional deviation of a wafer by detecting the entire circumference of the peripheral portion of the wafer by a CCD linear sensor while rotating the wafer mounted on the mounting table.
Further, for example, Patent Document 2 discloses a method of correcting a position of a wafer on tweezers by a transfer robot which is equipped with a positioning mechanism for the wafer (see, for example, FIGS. 2 and 3 of Patent Document 2). In this case, a positional deviation of the wafer is detected by detecting the entire circumference of the peripheral portion of the wafer by a light-receiving sensor disposed to face a light emitting diode, while rotating the wafer which is picked out by the tweezers and elevated by an elevating mechanism.    Patent Document 1: Japanese Patent Laid-open Application No. H8-8328    Patent Document 2: Japanese Patent Laid-open Application No. H5-343500    Patent Document 3: Japanese Patent Laid-open Application No. H11-91948    Patent Document 4: Japanese Patent Laid-open Application No. 2002-280287